It has long been known to produce products of the above described type, and currently such sandwich or laminated type products having a gypsum core are produced and sold as wallboard in a multibillion square foot market per year in the United States alone. Wallboard is made by mixing calcium sulfate hemihydrate with excess water, and sometimes fillers, to form a slurry which is deposited or cast between two layers of paper to form a sandwich. The hemihydrate hardens in place to form the dihydrate. The sandwich is moved on a conveyor to a long drying oven, where excess water is removed. The wallboard making machines are very large and expensive and the art has long desired to speed up the drying of the sandwich and hence the output of the machines.
Very few improvements have occurred in the wallboard manufacturing art. An early improvement of the product occurred when glass was employed as a reinforcement. This development is disclosed in U.S. Pat. No. 2,681,863. The patent indicates that a product having improved strength and fire resistance results.
It is an object of the present invention to increase the rate of drying of sandwiches comprising an inorganic core having layers of cellulosic paper on its surfaces, and, more particularly, to increase the throughput of wallboard making machines.
According to principles of the present invention, in has been discovered that wallboard which contains a random, three-dimensional, jack-strawed network, of glass monofilaments, having hydrophilic surfaces, which extends from within the core to the evaporative surface of the cellulosic paper skins will dry, by more than approximately 10%, faster than does wallboard in which the cellulosic paper contains no such network of monofilaments. Desirably, the individual glass monofilaments have a diameter of less than about 0.00080 inch and preferably will have a length of less than about 1 inch, e.g. a length of about 1/8 inch to about 1 inch and more desirably about one-quarter to about one-half inch.
As is well known in the art, textile glass fibers are produced by attenuating a plurality of monofilament glass streams issuing from a bushing into smaller diameter monofilaments which monofilaments have a size applied thereto. The sized monofilaments are then gathered into a strand, or bundle, of bound, parallel, individual monofilaments. In order to obtain the random, three-dimensional, or jack-strawed, network of monofilaments for use herein, the size employed should be water soluble, or water dispersible, i.e., the size in general should be hydrophilic, so that the size will allow the individual monofilaments making up the bundles, or strands, to be readily dispersed with mixing in water but without substantial reduction in monofilament length. Numerous water soluble and water dispersible sizes are well known in the art.
It is know that nascent glass has an angle of wetting that approaches zero, and it is theorized that spreading of water along the glass surfaces accounts for the drying effect of the present invention. To accomplish this drying effect, the monofilaments should be touching or so nearly touching that the water can spread along their surfaces, all of the way from the core to the exposed surface of the cellulosic paper where evaporation takes place. Suitably, in order to obtain the water transmitting relationship to effect improved drying, the monofilaments will be present in a jack-strawed arrangement in the cellulosic paper in an amount of about 0.02% to about 4% by weight of the paper but most desirably in an amount of about 0.05%-1.0%. Outstanding results will be realized when the amount is about 0.05% to about 0.3%. Similarily such water transmitting relationship will suitably be obtained by employing about 1,200 to about 20,000 lineal inches of monofilaments per cubic inch of cementitious core material.
In accordance with prior art practices starches are added to the inorganic cementitious slurries to bond the cellulosic paper skins to the core after it is dried. To accomplish this the starch is solvatable to a degree wherein it migrates with the excess water through the core to be adjacent the cellulosic paper skin.